On Wed, Mar 25, 2020 at 4:32 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Mar 25, 2020 at 03:43:02PM +0100, Miklos Szeredi wrote: > > > > > > - while ((page = readahead_page(rac))) { > > > - if (fuse_readpages_fill(&data, page) != 0) > > > + nr_pages = min(readahead_count(rac), fc->max_pages); > > > > Missing fc->max_read clamp. > > Yeah, I realised that. I ended up doing ... > > + unsigned int i, max_pages, nr_pages = 0; > ... > + max_pages = min(fc->max_pages, fc->max_read / PAGE_SIZE); > > > > + ia = fuse_io_alloc(NULL, nr_pages); > > > + if (!ia) > > > return; > > > + ap = &ia->ap; > > > + __readahead_batch(rac, ap->pages, nr_pages); > > > > nr_pages = __readahead_batch(...)? > > That's the other bug ... this was designed for btrfs which has a fixed-size > buffer. But you want to dynamically allocate fuse_io_args(), so we need to > figure out the number of pages beforehand, which is a little awkward. I've > settled on this for the moment: > > for (;;) { > struct fuse_io_args *ia; > struct fuse_args_pages *ap; > > nr_pages = readahead_count(rac) - nr_pages; > if (nr_pages > max_pages) > nr_pages = max_pages; > if (nr_pages == 0) > break; > ia = fuse_io_alloc(NULL, nr_pages); > if (!ia) > return; > ap = &ia->ap; > __readahead_batch(rac, ap->pages, nr_pages); > for (i = 0; i < nr_pages; i++) { > fuse_wait_on_page_writeback(inode, > readahead_index(rac) + i); > ap->descs[i].length = PAGE_SIZE; > } > ap->num_pages = nr_pages; > fuse_send_readpages(ia, rac->file); > } > > but I'm not entirely happy with that either. Pondering better options. I think that's fine. Note how the original code possibly over-allocates the the page array, because it doesn't know the batch size beforehand. So this is already better. > > > This will give consecutive pages, right? > > readpages() was already being called with consecutive pages. Several > filesystems had code to cope with the pages being non-consecutive, but > that wasn't how the core code worked; if there was a discontiguity it > would send off the pages that were consecutive and start a new batch. > > __readahead_batch() can't return fewer than nr_pages, so you don't need > to check for that. That's far from obvious. I'd put a WARN_ON at least to make document the fact. Thanks, Miklos